In the remainder of this document, the term “signal” is used to denote an analog or digital signal, and a digital signal will be treated the same way as an analog signal.
More specifically, the invention relates to such a device for measuring time at very high precision, known in the state of the art, based on the use of a timed counter by a clock, and associated with a delay line with a plurality of basic delay elements, for example equal in length. For example, such a delay line may contain delay elements D for a period equal to td.
The sum of the elementary delays D×td can be controlled by the clock period. In another case, it must be regularly calibrated.
Additional elementary delays are sometimes used, most often to compensate for side effects.
Thus, and in such a device, the input signal is firstly converted into a binary pulse by converter means, for example based on the use of a discriminator.
In its simplest embodiment, this discriminator can for example be a simple comparator, comparing the input signal to a set threshold value. However, to obtain correct dating, this discriminator is often a more complex device, the propagation time of which must be thoroughly controlled.
The measurement of time is then performed on the binary signal issued by the discriminator by the assembly including the counter and the delay line.
When the pulse reaches the discriminator output, or upon a subsequent clock edge, the counter and the delay line are either stopped or recopied into another record or storage element.
The delay line then gives the thin part of the measurement time within the clock period in addition to the counter.
However, this device has two major drawbacks, namely:
the discriminator has a reaction time depending on the shape and amplitude of the input signal and the discrimination threshold, and
the time measurement pitch is that of the delay line, limited to the first order by the microelectronic technology chosen for the corresponding circuit.
Several improvements to this type of device have already been proposed in the state of the art.
Thus, for example and with regard to the discriminator, the use of a constant fraction discriminator makes it possible to make the first order the propagation time independent from the signal amplitude. Nevertheless, this only works on amplitude-homothetic signals. In addition, the integration of such a discriminator in a circuit of the ASIC type has great difficulties.
The improvements related to the delay lines call mostly for an interpolation among several of the latter. Although these solutions provide a significant gain in precision, they also have a number of drawbacks, including:
a complexity of their operating principle and practical execution,
an increase in the surface area required for the physical implementation and consumption related to the additional delay lines, and
the general need for heavy and frequent calibration.
It will be noted that it is possible to minimize downtime associated with the acquisition of such a device for example by replacing the records in which the states of the counter and the delay line are copied with double port storage of the FIFO type, for example. The integration of multi-path microelectronics of this type of devices is carried out conventionally.
In practice, the resolution of this type of devices is limited to a few tens of picoseconds rms.
Alternatively, the thin timing part of the measurement device can be replaced with a time amplitude converter made by a voltage ramp generator started by the output of the discriminator and stopped by the clock. This voltage digitized by an analog-digital converter (ADC) is an image of the thin time. The thin dating pitch of the device here only depends on the slope of the ramp and the ADC pitch. The speed of the microelectronic technology used does not appear here to be a limitation to the resolution.
On the contrary, the resolution of this type of device is limited by the noise on the ramp, its linearity, and the precision (and artefacts) of starting and stopping of the ramp generator. Therefore, in its integrated version, the temporal resolution (of the measuring device only) is at best in the vicinity of 10 ps rms. This implementation is on the contrary adverse in terms of downtime and even requires a calibration that could be complex.